In addition to a function of uninterruptedly supplying power to a user, a UPS also has functions of improving power quality and providing high power quality. The UPS generally uses a double-conversion working mode. When the UPS works, the UPS needs to consume electric energy, and greater output power of the UPS indicates more consumed electric energy. For the purpose of energy saving, when the quality of a power grid is good, a concept of working in an economical operation mode (ECO-mode) is proposed, that is, the power grid provides mains power for a load of a user directly through a bypass line (also referred to as a static bypass) of the UPS, while a main part of the UPS is in a standby state, its output power is zero, consumed electric energy is low, and therefore the efficiency of the entire system is high.
In the ECO mode, although the main part of the UPS is in the standby state, certain energy is still consumed, which hinders an effective improvement on the efficiency of the entire system in the ECO mode.